Bath for electrolytic deposition of magnetic films

ABSTRACT

A BATH FOR ELECTOLYTIC DEPOSITION OF MAGNETIC FILMS OF RELATIVELY HIGH, CONTROLLED COERCIVITY. THE BATH COMPRISES AN AMMONIACAL SOLUTION OF COBALT OR COBALT AND NICKEL SALT, TUNGSTEN AS TUNGSTIC ACID OR SODIUM TUNGSTATE AND SOLDIUM HYPOPHOSPHITE. THE CONCENTRATION OF SODIUM HYPOPHOSPHITE IN THE BATHE DETERMINES THE COERCIVITY OF THE RESULTANT MAGNETIC FILMS, AND PERMITS CONTROL THEREOF OVER A WIDE RANGE OF VALUES. ROCHELLE SALT OR OTHER CITRATE OR TARTRATE MAY BE USED IN THE BATH TO AID COMPLEXING OF THE COBALT, AND FORMALDEHYDE MAY BE ADDED TO INHIBIT ELECTROLESS DEPOSITION FROM THE BATH.

June 27, 1972 P. c. BALDWIN 3,672,968

BATH FOR ELECTROLYTIC DEPOSITION OF MAGNETIC FILMS Filed Feb. 2, 1970 g 700 33 E Q 600 h 500 E 00/0 from Table g x Dafa from Table 2 Lu 400 Q Q) 200 l i i l I i I I I SOD/UM HYPOPHOSPH/TE CO/VCE/Vffiflf/O/V 2 (Grams/Lifer) INVENTOR.

PH/L/P C. BALDWl/V b/ ATTORNEYS United States Patent 3,672,968 BATH FOR ELECTROLYTIC DEPOSITION 0F MAGNETIC FILMS Philip C. Baldwin, El Segundo, Calif., assignor to Burton Electrochemical Co., Inc., Los Angeles, Calif. Filed Feb. 2, 1970, Ser. No. 7,791 Int. Cl. C23b 5/32 U.S. Cl. 204-43 6 Claims ABSTRACT OF THE DISCLOSURE A bath for electrolytic deposition of magnetic films of relatively high, controlled coercivity. The bath comprises an ammoniacal solution of cobalt or cobalt and nickel salts, tungsten as tungstic acid or sodium tungstate and sodium hypophosphite. The concentration of sodium hypophosphite in the bath determines the coercivity of the resultant magnetic films, and permits control thereof over a wide range of values. Rochelle salt or other citrate or tartrate may be used in the bath to aid complexing of the cobalt, and formaldehyde may be added to inhibit electroless deposition from the bath.

BACKGROUND OF THE INVENTION (1) Field of the invention The present invention relates to a bath for electrolytic deposition of magnetic films. More particularly, the invention relates to a bath for electrolytic deposition of cobalt or cobalt and nickel magnetic films of relatively high, controlled coercivity, the bath containing both tungsten and sodium hypophosphite.

(2) Description of the prior art Large capacity data storage in computers often is facilitated by rotating discor drum-type magnetic memories. Such devices typically include a plated magnetic coating of relatively high coercivity and remanence. The high coercivity, generally in the range of from about 220 to about 700 oersteds, provides optimum data pulse resolution, while the high remanence maximizes the output signal level available from the memory device.

In the past, electroless deposition has been the usual method of plating magnetic alloy coatings onto disc and drum memories. Although various electroless plating baths are available which produce high coercivity mag netic coatings, certain deficiencies are inherent in electroless deposition. One such problem is the difficulty of predicting the value of coercivity which will be achieved with a particular electroless deposition bath, even though the process may be under close control. This problem is complicated by the occurrence of metal loss in the plating bath due to decomposition deposition. Concomitantly, there is no electroless bath available which will permit accurate control of the coercivity of the resultant magnetic coating over a Wide range of values.

Other shortcomings are associated with electroless deposition. For example, because of the autocatalytic nature of the process, considerable bath maintenance is required. To obtain high coercivity magnetic coatings by electroless deposition usually requires that the deposited layer be relatively thin, and thus susceptible to abrasion, scratching and the like. Further, it is somewhat difficult to control thickness of an electroless plated coating, and such coatings tend to be of relatively low hardness. These characteristics make it difficult to apply a wear-resistant layer of rhodium, chromium or the like over the magnetic coating.

Patented June 27, 1972 In contrast to electroless deposition, electrolytic plating processes characteristically are more stable and reliable. Electrolytic coatings are of quite predictable thickness, since plating rate and thickness are controlled by the plating low coercivity nickel-iron-phosphorous films. In electroless deposits since alloy composition can be controlled over a wider range than possible using autocatalytic methods. Further, electroplated coatings are receptive to the direct application of rhodium, chromium or like films, added to enhance the wear-resistance of the plated magnetic layer.

In the past, it has not been possible to realize the potential advantages of electroplated magnetic coatings because of the limited range of coercivities which could be obtained. For example, US. Pat. No. 3,202,590 to Koretzky teaches electrolytic deposition of a high coercivity, cobalt-phosphorous magnetic coating using an acidic bath containing only sodium hypophosphite. However, very little, if any control over coercivity of the resultant coating is possible with such a bath. As described in another US. Pat. No. 3,119,753 to Mathias, an alkali metal hypophosphite is incorporated in a bath for electroplating low-coercivity nickeLiron-phosphorous films. In this bath, the hypophosphite is used to reduce the coercivity and to control the characteristics of the plated films, which typically had coercivity values on the order of less than 10 oersteds.

Another approach of the prior art is shown in U.S. Pat. No. 3,152,974 to V. Zentner. As shown therein, high coercivity cobalt-nickel films may be deposited from baths using either phosphorous acid or tungsten (as sodium tungstate) but not both. During the deposition process, simultaneous AC and DC excitation is used. Again, only very limited control on the coercivity of the deposited films is possible.

The foregoing and other shortcomings of the prior art are overcome by using the inventive bath for electrolytic deposition of magnetic films. The bath permits electrolytic deposition of high coercivity magnetic films of greater thickness and hardness than films of like coercivity deposited autocatalytically. Moreover, the films exhibit high remanence, and most important, the coercivity of the deposited film can be accurately controlled over a wide range of values. The bath is stable, with no tendency for electroless deposition or decomposition deposition therefrom, and permits plating using only DC excitation.

SUMMARY OF THE lN'VENTION In accordance with the present invention, there is set forth a bath for electrolytic deposition of magnetic films of relatively high, controlled coercivity. The bath comprises an ammoniacal solution of cobalt or cobalt and nickel salts, tungsten as tungstic acid or sodium tungstate and sodium hypophosphite. The concentration of sodium hypophosphite in the bath determines the coercivity of the resultant magnetic films, and permits control thereof over a range of valves extending from about 250 to about 700 oersteds. Such control over coercivity is not possible using sodium hypophosphite alone, but represents a synergistic effect of tungsten plus sodium hypophosphite.

In a preferred embodiment, the plating bath is adjusted to have a pH on the order of 10, to assist solubility of the tungstic acid or sodium tungstate. A tungstic acid concentration of 20 grams per liter of bath was found to produce optimal magnetic films, but a wide range of tungsten concentration is acceptable. Complexing of the cobalt and nickel in the solution is aided by the use of Rochelle salt or other citrate or tartrate. Formaldehyde may be added to the bath to prevent electroless disposition therefrom. For maximum remanence, the cobaltnickel ratio in the solution is maintained at about to 1, however, considerable variation in this concentration still produces acceptable magnetic films.

Thus, it is .an object of the present invention to provide an improved bath for electrolytic deposition of magnetic films.

Another object of the present invention is to provide a bath for electrolytic deposition of cobalt or cobalt-nickel films of relatively high, controlled coercivity.

It is another object of the present invention to provide a bath for electrolytic deposition of magnetic films, the bath including both tungsten as tungstic acid or sodium tungstate and sodium hypophosphite, the sodium hypophosphite concentration determining the coercivity of the deposited film.

Still another object of the present invention is to provide a bath for electrolytic deposition of magnetic films, the bath comprising an ammoniacal solution of cobalt or cobalt and nickel salts, tungsten and sodium hypophosphite.

It is a further object of the present invention to provide a bath for electrolytic deposition of cobalt or cobalt and nickel films, the films including relatively small concentrations of both tungsten and phosphorous, such films being particularly useful for rotating magnetic memory applications.

Yet another object of the present invention is to provide a bath for electrolytic deposition of cobalt or cobaltnickel films, the bath including, formaldehyde to inhibit electroless deposition therefrom.

BRIEF DESCRIPTION OF THE DRAWINGS Still other objects, features and attendant advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description of the preferred embodiments constructed in accordance therewith, taken in conjunction with the accompanying drawings wherein like numerals designate like parts in the several figures and wherein:

FIG. 1 is a fragmentary sectional view of a disc memory incorporating a magnetic coating electrolytically deposited from the inventive plating bath; and

FIG. 2 is a graph showing coercivity of a magnetic film electrolytically deposited from the inventive bath as a function of sodium hypophosphite concentration in the bath.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, and particularly to FIG. 1 thereof, there is shown a fragmentary sectional view-of a disc-type magnetic memory for computer or related applications, and utilizing a magnetic film electrolytically deposited in accordance with the present invention. Specifically, a disc memory device includes a metal substrate 12 typically fashioned of aluminum, magnesium or the like. As well known in the rotating memory art, substrate 12 typically may be on the order of A" thick by several feet in diameter, device 10 being provided with an appropriate coaxial mounting and drive mechanism for rotation at a constant speed beneath one or more conventional read or write heads (not shown). Disposed atop substrate 12 may be a thin layer 14 of a non-magnetic metal such as copper, gold, nickel or the like.

Disposed atop layer 14 is a magnetic film 16 prepared by electrolytic deposition from a plating bath in accordance with the present invention. Magnetic layer 16 typically will exhibit relatively high coercivity and remanence. Generally, magnetic layer 16 will have a thickness of from about 10 microinches to about 200 microinches. To enhance the wear-resistance of disc memory 10, a layer 18 of rhodium, chromium or similar hard, nonmagnetic material is disposed atop magnetic film 16. Thus, wear- .4 resistant layer 18 is exposed to the abrasive forces of a read or write head which might be in physical contact with disc memory 10, thereby preventing damage to magnetic layer 16.

For optimum performance, magnetic film 16 should exhibit relatively high coercivity and remanence. These characteristics are achieved by electrolytically depositing magnetic layer 16 from a plating bath in accordance with the present invention. In general, the inventive bath comrises an ammoniacal solution of cobalt or cobalt and nickel salts, tungsten as tungstic acid or sodium tungstate and sodium hypophosphite. The concentration of sodium hypophosphite in the bath determines the coercivity of the resultant magnetic film, and permits control thereof over a wide range of values.

Use of an ammoniacal solution is mutually compatible with requirements for cobalt and nickel complexing and for solubility of tungstic acid (also known as tungstic oxide) or sodium tungstate. Accordingly, a pH greater than about 8 is desired for the bath, a pH value of 10 being I optimal for solubility of the tungsten compound.

Cobalt is supplied to the bath as a cobalt salt, preferably as cobalt sulfate but alternatively as cobalt chloride, cobalt bromide or cobalt iodide. Rochelle salt or other citrate or tartrate may be added to the bath to aid complexing of the cobalt. A considerable range in cobalt concentration in the bath is acceptable, however, the preferred range of cobalt (as metal) is from about 7 to about 20 grams per liter. A value of 12 grams per liter represents an economically acceptable cobalt metal level in the bath, and provides excellent magnetic films.

Nickel may be added as a nickel salt, such as nickel sulphate hexahydrate (NiSO -6H O). The relative nickel concentration will determine the cobalt-nickel ratio in the resultant magnetic film. It has been found that the remanence of the resultant magnetic films increase somewhat with increasing nickel concentration up to a cobaltnickel ratio of about 5 to l; at higher nickel concentrations the remanence decreases slightly. The coercivity control provided by the sodium hypophosphite concentration is independent of the amount of nickel in the bath. Accordingly, the invention encompasses plating solutions in which no nickel is present, as well as baths to which con-, siderable nickel has been added.

To obtain control of the coercivity of the deposited magnetic films over a wide range of values, both tungsten (as tungstic oxide or sodium tungstate) and sodium hypophosphite should be present in the bath. If no tungsten is present, very little increase in coercivity is obtained as the sodium hypophosphite level in the bath is increased. With no sodium hypophosphite present, the addition of tungsten alone increased coercivity only slightly. The combined elfect of tungsten and sodium hypophosphite is synergistic.

A tungstic oxide concentration of about 20 grams per liter of bath was found to be optimum for the synergistic elfect just described. However, the effect is still significant with a tungstic oxide concentration of 10 grams per liter, and the effect did not increase significantly with a concentration of 40 grams per liter. Since tungstic oxide is quite expensive, economic considerations suggest using the optimum concentration of about 20 grams per liter. With sodium tungstate, optimum concentration is about 14 grams per liter.

With sodium hypophosphite present in the electroplating bath in considerable amounts, there is a tendency for electroless deposition to occur. Such autocatalytic deposition is prevented by the addition of a small amount of formaldehyde into the plating bath. For example, the addition of 15 milliliters of 37% formaldehyde per liter of bath solution was found to inhibit electroless deposition at the typical operating temperature of F. Considerably more formaldehyde can be added with no adverse effect whatever on the magnetic perameters of the deposited film. For a particular bath, the minimum amount of formaldehyde needed to prevent electroless deposition is readily determinable.

With tungsten present in the plating bath, the amount of sodium hypophosphite added determines the coercivity of the resultant magnetic film. This effect is illustrated by the graph of FIG. 2 for a typical plating bath in accordance with the present invention. While the values plotted in FIG. 2 apply to the baths described in detail hereinbelow in conjunction with Tables 1 and 2, the illustrated relationship between sodium hypophosphite concentration and coercivity is typical of that obtained with baths having different ingredient concentrations. As indicated in FIG. 2, for the exemplary baths described, the film coercivity is controllable over a range of from about 220 to about 700 oersteds, with increasing sodium hypophosphite concentration yielding increased coercivity.

A number of bath solutions were prepared to illustrate the effect of sodium hypophosphite concentration on the coercivity of the resultant magnetic films. To accomplish this, a stock solution was prepared which contained the cobalt, dissolved tungstic acid and formaldehyde. This stock solution was filtered, diluted to operating concentration and used as needed. For each test sample, an appropriate amount of nickel and an incremental amount of sodium hypophosphite was added.

To prepare a 1400 ml. batch of stock solution, a Pyrex beaker was placed on a heated magnetic stirrer plate. Approximately 800 ml. of hot, deionized water was placed in the beaker, followed by 300 grams of Rochelle salt and 50 ml. of 28% ammonium hydroxide. Approximately 30 grams of tungstic oxide powder then was added and the mixture was stirred at a temperature of between 140 and 160 F. for a sufficient period of time (approximately 15 minutes) to dissolve the tungstic oxide. After the tungstic oxide was dissolved, 50 grams of cobalt sulfate hexahydrate was added; stirring continued to dissolve the cobalt sulfate. Sufficient additional ammonia was added to raise the pH of the bath to 10. Finally, 20 ml. of 37% concentration formaldehyde was added, and the stock solution filtered prior to use. Cobalt (as metal) represented approximately 12.0 grams per liter of the stock solution.

To illustrate the relationship between sodium hypophosphite concentration and coercivity of a cobalt plating bath in accordance with the present invention, various amounts of sodium hypophosphite were added to the stock solution just described, and a test sample plated and tested for each formulation. To accomplish plating, the stock solution including sodium hypophosphite was placed in a Pyrex beaker provided With titanium-platinum metal anodes. Stirring and temperature control were facilitated with a magnetic stirrer-hot plate combination. Electroplating was performed with a small direct current rectifier, the beaker being sufficiently large to accommodate a standard brass Hull-Cell panel.

Prior to plating, the standard brass Hull-Cell panels were marked and given a preliminary copper strike. The panel then was Weighed as a preliminary step to measuring the thickness of the deposited magnetic film by standard weight-gain techinques. The samples were cleaned and activated conventionally. Plating for the tests was performed at a bath temperature of 120 F. with mild stirring. A current of 1 ampere was applied for 5 minutes; this corresponds to a current density of 6.9 amperes per square foot.

Magnetic measurements were made on a standard 60 hertz B-H loop tester using inch by 4 inch test strips, out inch in from the edge of the panel. The outer strip was discarded to eliminate the edge effect characteristics of electroplating processes. Magnetic measurements were made at an applied field strength of 1,000 oersteds which is standard for industry measurements of this type.

Table 1 below shows the effect of varying sodium hypophosphite levels on a system containing approximately 12 grams per liter of cobalt (as metal) but no nickel.

As may be seen from Table 1, the magnetic film electro lytically deposited on each of samples I through V had an average thickness of about 27 microinches and exhibited a square hysteresis loop shape. The remanence of each of samples I through V was on the order of 5,000 gauss. The coercivity of the samples was dependent on the sodium hypophosphite concentration.

Sample I, which was deposited from a bath containing tungsten (as tungstic oxide) but no sodium hypophosphite, exhibited a coercivity of 220 oersteds. Sample V, deposited from a bath having sodium hypophosphite concentration of 43.0 grams per liter, exhibited a coercivity of 500 oersteds. Intermediate these values, different concentrations of sodium hypophosphite resulted in the related coercivity values listed in Table 1.

The data set forth in Table 1 was used to plot the points designated 0 in FIG. 2. Accordingly, the curve of FIG. 2 illustrates the relationship between sodium hypophosphite concentration and the resultant coercivity of samples I through V.

To indicate the effect of sodium hypophosphite concentration in a cobalt-nickel system, sufiicient nickel sulfate hexahydrate was added to the stock solution described hereinabove to provide a nickel (as metal) concentration of 2.4 grams per liter. This corresponded to a cobaltnickel ratio in the plating bath of approximately 5 to 1. Table 2 below sets forth the measured coercivity values for samples designated VI through XIII prepared from such cobalt-nickel baths having different sodium hypo phosphite levels.

TABLE 2 Average Sodium Hyster- Weight thickness, hypophos- Coercwesrs gain, microhite, rty, loop grams inches grams/liter oersteds shape 074 24 0 220 S quare .078 26 7.1 310 D0. 065 21 21. 3 390 D0. 098 32 43. 0 510 D0. 094 31 50. 0 550 D0. 091 30 60.0 620 D0. 087 28 75. 0 680 Do.

As readily may be seen from Table 2, the coercivity of the electrolytically deposited cobalt-nickel films varied from about 220 oersteds with no sodium hypophosphite present in the plating bath to about 680 oersteds with about 75 grams per liter sodium hypophosphite present. The data listed in Table 2 are designated X in FIG. 2. It is interesting to note that the sodium hypophosphite concentration produces coercivity value in this cobaltnickel system which correspond closely to the values obtained in the system containing only cobalt. In each case it is quite clear that the sodium hypophosphite concentration in the bath containing tungsten controls the coercivity of the resultant electroplated magnetic film.

The remanence of the samples listed in Table 2 were found to lie in the range of from about 8000 to about 7200 gauss, decreasing in value as coercivity increased to about 550 oersteds. At higher coercivity values, the remanence dropped to a range of between about 5000 and 6000 gauss.

Although not tabulated herein, tests also were made with different cobalt-nickel ratios in the bath. These tests indicated that the coercivity still was determined by the sodium hypophosphite concentration in the electrolytic plating bath. The remanence values of the resultant films were found to increase generally with increasing nickel concentration up to a cobalt-nickel ratio of about to 1 (represented by the samples of Table 2). With increase in nickel concentration above that level, a slight decrease in remanence was noted.

Although the above-described samples were plated at a bath temperature of 120 F. with a current density of 6.9 amperes per square foot, it is to be understood that the invention is not so limited. Rather, the bath is operative over a wide range of temperatures, and a wide range of current densities. However, because tungsten is present in the plating solution, it is desirable that a minimum current density of about 2 amperes per square foot be used. Moreover, although not required, it is desirable that the bath be stirred gently during deposition to insure uniformity of bath solution and to minimize streaking.

Although titanium-platinum electrodes were used to prepare the samples described, this is not required and other materials, such as stainless steel may be used for the plating anodes. With stainless steel electrodes it is preferable to use cobalt sulfate as the cobalt salt; titanium or platinum electrodes also permit use of cobalt chloride or other cobalt salts.

It should be noted that the magnetic films electrolytically deposited from the inventive bath will contain small amounts of tungsten and phosphorous in addition to cobalt and (if used) nickel. Thus, tungsten may be present in the alloy up to with phosphorous also being present in amounts up to 10%. Of course, the actual amounts of tungsten and phosphorous present in the deposited alloy will be determined by the concentration of these elements in the plating bath itself, and in this regard, the phosphorous concentration appears related to the coercivity of the resultant film.

The present invention also encompasses coercivity control when using a cobalt-iron or cobalt-nickel-iron electrolytic deposition bath. However, the presence of iron in the bath reduces the coercivity of the resultant films somewhat below the values shown in FIG. 2. When iron is present in amounts up to .15 gram per liter, slight improvement in hysteresis loop squareness is obtained.

While the invention has been described with respect to the preferred physical embodiments constructed in accordance therewith, it will be apparent to those skilled in the art that various modifications and improvements may be made without departing from the scope and spirit of the invention.

-I claim: it

1. In a plating bath of the type including a cobalt salt in an aqueous, ammoniacal solution and useful for electrolytic deposition of a magnetic film, the improvement comprising: including in said bath at least 10 grams per liter of a tungsten compound and sodium hypophosphite in an amount sufiicient to raise the coercivity of a cobalt magnetic film deposited from said bath, said cobalt salt being selected from the class consisting of cobalt sulfate, cobalt chloride, cobalt bromide and cobalt iodide, said cobalt being present in concentration of from about 7 to about 20 grams per liter, and further comprising sutficient formaldehyde to prevent electroless deposition from said bath.

2. A plating bath as defined in claim 1 further comprising a nickel salt.

3. A plating bath as defined in claim 2 wherein said nickel salt comprises nickel sulphate hexahydrate, and wherein said cobalt-nickel ratio is on the order of 5:1.

4. A plating bath as defined in claim 1 wherein said tungsten compound is selected from the class consisting of tungstic acid and sodium tungstate.

5. A plating bath as defined in claim 1 further comprising a complexing agent selected from the class consisting of citrates and tartrates.

6. An electrolytic plating bath comprising an aqueous, ammoniacal solution containing cobalt or cobalt and nickel complexes, the concentration of cobalt being between about 7 and 20 grams per liter, at least 10' grams per liter of tungstic acid or sodium tungstate, sodium hypophosphite in an amount sufiicient to raise the coer civity of a cobalt magnetic film deposited from said bath, and sufficient formaldehyde to prevent electroless deposition therefrom.

References Cited UNITED STATES PATENTS 3,485,597 12/ 1969 Pearlstein 106-1 X 3,485,725- 12/1969 Koretzky 204-48 X 3,227,635 1/1966 Koretzky et al 20448 X 3,578,468 5/1971 Pearlstein et a1 106-4 OTHER REFERENCES Metal Finishing, p. 43, November 1969.. H. Koretzky, IBM Tech. Disclosure Bulletin, vol. 9, No. 1.1, p. 1634 (1967).

GERALD L. KAPLAN, Primary Examiner 

